Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of inkjets, which are arranged in one or more printheads, onto an image receiving surface. In an indirect inkjet printer, the printheads eject ink drops onto the surface of an intermediate image receiving member, such as a rotating imaging drum or belt, and the image is later transferred and fixed to the media. In direct to media printers, the printheads eject ink drops directly onto the media and the image is later fixed to the media. In both types of printers, the printer forms an image by generating and delivering firing signals to printheads that operate the inkjet ejectors within the printheads. These firing signals are generated with reference to digital image data. The operation of the inkjet ejectors expels individual ink drops from the inkjets that land at particular locations on the image receiving member. The locations where the ink drops land are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
During printing, the printheads and the image receiving surface move relative to one other and the inkjets eject ink drops at appropriate times to form an ink image on the image receiving surface. The ink ejected from the inkjets can be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, some inkjet printers use phase change inks that are loaded in a solid form and delivered to a melting device. The melting device heats and melts the phase change ink from the solid phase to a liquid that is supplied to a printhead for printing as liquid drops onto the image receiving surface.
Operating the inkjets in the printheads at different frequencies causes the inkjets to eject ink drops of different masses. During printer manufacture, printheads are set up, through modifications of firing voltages and waveforms, to produce a default drop mass at the maximum rate of operation of the printhead. Because the mass of ink drops varies considerably with frequency at lower rates of operation, printheads may also be setup in the factory to eject drops at a lesser predetermined mass when the inkjets are operated to form a 25% pattern, which activates the inkjet ejectors at a rate that is one-quarter of the maximum frequency of the printhead. Although these calibrations help attenuate image quality issues occurring from widely different ink masses being ejected at different frequency rates, differences still occur because the data used to generate the firing signals operate inkjet ejectors at non-periodic rates. While these differences have no appreciable effects on image quality, they do affect the accuracy of ink usage estimation schemes implemented in printers.
Estimating ink usage is important to printer users so they can determine the costs of printer operation and schedule their supply purchases. Typically, a controller in a printer is programmed to estimate ink usage with reference to some usage model based on the colors in the original images produced by the printer. Some estimating programs process the contone image data, while others count the number of drops ejected by the printheads. As noted above, the rate of operation of an inkjet affects the mass of ink drops ejected by the inkjet. Estimating or accurately measuring the amount of ink used to produce a print job enables a printer to allocate appropriately the cost of the ink used to produce the print job for customers. Thus, ink usage estimates would be improved by taking the variations in ejected ink drop masses into account.